1. Field of the Invention
The present invention relates to a temperature control method for use in a fixing apparatus for thermally fixing a toner image onto a recording sheet of a copying machine or printer and, more particularly, to a temperature control method for a fixing apparatus in which a pair of rolls abut against the front and back of the recording sheet and are individually equipped with heaters.
2. Description of Related Art
In most of the fixing apparatus of this kind in the prior art, the heater is packaged in only that one of the paired rolls to be brought into abutment against the front surface of the recording sheet carrying a toner image thereon. Moreover, the temperature control of the heater is always adjusted to a target value of the surface temperature of the roll by setting a certain target temperature in advance and by energizing the heater only when the surface temperature of the roll having the heater packaged therein becomes lower than the target value.
In the so-called "full-color copying machine", however, an unfixed toner image is formed by overlapping toners of four colors of Cyan, Magenta, Yellow and Black. If, therefore, the unfixed toner image is heated only from the front side of the recording sheet, there arises a problem that the fixing property is deteriorated because sufficient thermal energy will not propagate to the toner of the lowermost layer contacting with the recording sheet. On the other hand, the full-color recorded image itself requires more sufficient thermal energy to be applied to the toners than a monochromatic recorded image, if its coloring property is considered.
For use in the full-color copying machine or the like, therefore, there has been proposed in recent years a fixing apparatus which is equipped with a heater even in the roll to be brought into abutment against the back side of the recording sheet.
However, this fixing apparatus having the individual heaters in the rolls abutting against the front and back sides of the recording sheet has to consider as its important problem how the temperatures of the individual rolls are to be controlled. This is because the copying machine or printer, in which the fixing apparatus of that kind is to be packaged, is designed premising that it is used with the home-service power supply of 100 V, so that the simultaneous energizations of the two heaters cannot be achieved because of the restrictions on the rated current. Thus, what can be energized is only one heater. The specific temperature control methods proposed in the prior art will be described in the following together with their problems.
In a first method, two rolls are individually set with target temperatures so that one of the roll has its heater energized if its surface temperature only is lower than the target value or so that the two rolls have their individual heaters energized alternately for a constant time period if both their surface temperatures are lower than the target values.
However, the efficiency for the rolls abutting against the front and back sides of the recording sheet to contribute to the fixing of the unfixed toner image, that is, the thermal energies for the individual rolls to be applied to the toners for a constant time period are naturally higher at the front-side roll contacting directly with the unfixed toner image than at the back-side roll. In case, therefore, the roll is liable to have its surface temperature seriously dropping as in the continuous copying operation, the thermal energy to be stored in the individual rolls could be more efficiently used if the heater of the front-side roll were preferentially energized. In this connection, according to this control method, an equal quantity of thermal energy is applied to the rolls abutting against the front and back sides of the recording sheet so that an excess thermal energy has a tendency to be stored in the roll abutting against the back side. Thus, this method is accompanied by a problem that the restricted thermal energy cannot be efficiently used.
In view of the problems of the first method, there has been proposed a second method, in which the heater of the front-side roll is preferentially energized if both the surface temperatures of the two rolls are lower than the target values. In other words, the heater of the back-side roll is not energized so long as the surface temperature of the front-side roll fails to reach the target value.
In the fixing device having both its front- and back-side rolls equipped with the heaters, however, a sufficient fixing property might be achieved if the surface temperature of the back-side roll were at a considerably high level although that of the front-side roll were lower than the target value. According to this control method, therefore, the energy efficiency is seriously deteriorated because the front roll is heated more than necessary.
Since, moreover, the surface temperatures of the rolls could not reach the target values if in the continuous copying operations, according to this control method, the heater of the front-side roll is kept energized once the copying operations are started, till the copying operations are ended so that the surface temperature of the front-side roll restores its target value. As a result, this method is troubled by a problem that the temperature in the inside of the front-side roll rises to a level higher by about 50.degree. C. than the target value till the heater is turned off, thus causing the so-called "overshoot phenomenon", in which the surface temperatures of the rolls exceed the target values by 20.degree. C. to 30.degree. C.
Incidentally, the fixing apparatus of the prior art is equipped with control means for deciding the incapability of the fixing operation in relation to the temperature control of the rolls having the heaters, if the roll surface temperatures drop to levels lower than the lowest fixing temperature determined in advance. In the fixing apparatus, however, which is equipped with the individual heaters in the rolls to be brought into the front and back of the recording sheet, as has been described hereinbefore, the lowest temperature required for the front-side roll to reproduce the predetermined fixing property would change with a change in the surface temperature of the backside roll. It would, therefore, be seriously diadvantageous to decide the capability of the fixing operation in terms of the lowest constant fixing temperature, as in the prior art. Specifically, the actually capable state of the fixing operation might be misjudged to be incapable, or vice versa. Thus, it has been impossible to avoid the reduction in the operating efficiency of the apparatus and the deterioration of the image fixing property.
Thus, in the temperature control methods of the prior art, the wasteful thermal energies having no contribution to the fixing of the unfixed toner image are stored in the individual rolls to deteriorate the energy efficiency, and still the worse the apparatus itself is adversely affected by the overshoot phenomenon. Moreover, the decision of the capability of the fixing operation is not accurate to deteriorate the operating efficiency of the copying machine or printer and the reliability of the fixing operation.